1. Field of the Invention
The present invention relates to a motor stator, a spindle motor including the motor stator, and a disk drive including the spindle motor.
2. Description of the Related Art
Disk drives such as hard disk drives use brushless motors (hereinafter, simply referred to as motors). Recently motor control methods are changing from linear control conventionally used to PWM (pulse width modulation) control which can effectively reduce power consumption. In PWM control, a pulsed current is input to a motor so as to cause generation of a counter-electromotive force by the motor inductance. The counter-electromotive force is used for driving the motor, thereby reducing a power required for driving the motor. Therefore, it is very important to minimize variations in the motor inductance.
When the pulse width of the pulsed current input to the motor is controlled in accordance with the rotation speed of the motor, highly efficient control can be achieved. In linear control, the rotation speed of the motor is obtained based on the counter-electromotive force. When the rotation speed is low, however, the counter-electromotive force is small and it is therefore difficult to obtain the rotation speed. For this reason, linear control cannot provide good controllability. On the other hand, in PWM control, a detected inductance value is used for improving rotation control performance in the low rotation speed range. Thus, it is important to make the inductances substantially even in PWM control.
In recent years, areal density of hard disk drives has rapidly increased. In the high-density hard disk drives, even a small amount of contamination causes failure. Therefore, it is strongly demanded to keep the inside of the hard disk drive clean in order to avoid contamination.
In motor stators, two-layer self-bonding magnetic wires have been used for forming coil windings. The two-layer self-bonding magnetic wire includes a magnetic wire coated with insulating material, e.g., polyurethane, polyester, and a bonding layer of thermoplastic resin or thermosetting resin, e.g., polyamide resin, butyral resin, epoxy resin. The two-layer self-bonding magnetic wire is usually heated after being wound around teeth of the motor stator, in order to prevent loosening of the two-layer self-bonding magnetic wire.
However, heating of the two-layer self-bonding magnetic wire causes generation of gas. The thus generated gas stays within the motor stator and leaks to the inside of a hard disk drive after the motor stator is mounted in the hard disk drive. Therefore, in order to keep the inside of the hard disk drive clean, change of the self bonding magnetic wire to non-self-bonding wire which does not have a bonding layer.
However, the non-self-bonding wire can be loosened easily and therefore requires an additional winding sequence in order to prevent the loosening. This makes mutual inductances between phases in a motor stator different. Especially in high-speed motors, it is necessary to use a conductive wire having a larger diameter and reduce the number of turns of winding of the conductive wire in order to make counter-electromotive force smaller. Therefore, effects of one or two turns for preventing the loosening become larger.
In order to prevent the loosening of the non-self-bonding wire, there is a technique of attaching a ring made of synthetic resin to the motor stator and catching the non-self-bonding wire with a projection of the ring. However, this technique increases the number of components which increases the cost. The increase in the number of components also makes the motor stator larger, resulting in the size increase of the hard disk drive. Thus, the use of the ring of synthetic resin is not preferable.